[stellar]

12 volts will really make it zing. Don't run vey long with 12 volts. If it is a big 6 volt charger with a high amp function like boost, that would be a good way to test. Set the charger to high. Some of the older chargers had a volt and an amp meter on them which could give you a better idea of the condition of the starter. 12 volts will give them little electrons a bigger push which could mask some problems, but 12 volts won't hurt anything for a quick test. Years ago i used to jump my 55 international 6 volt truck from a 12 volt car with no problems. Being that the starter is off the machine running on 12v will make it spin very fast.